Cubic Perovskite Structure of Black Formamidinium Lead Iodide, α-[HC(NH2)2]PbI3, at 298 K

The structure of black formamidinium lead halide, alpha-[HC(NH2)(2)]PbI3, at 298 K has been refined from high resolution neutron powder diffraction data and found to adopt a cubic perovskite unit cell, a = 6.3620(8) angstrom. The trigonal planar [HC(NH2)(2)](+) cations lie in the central mirror plane of the unit cell with the formamidinium cations disordered over 12 possible sites arranged so that the C-H bond is directed into a cube face, whereas the -NH2 groups hydrogen bond (NH center dot center dot center dot I = 2.75-3.00 angstrom) with the iodide atoms of the [PbI3](-) framework. High atomic displacement parameters for the formamidinium cation are consistent with rapid molecular rotations at room temperature as evidenced in ab initio molecular dynamic simulations.